1. Field of the Invention
The present invention relates to a domain-wall-displacement-type magnetooptical recording medium suitable for ultra-high-density recording utilizing displacement of domain walls during a reproducing operation. More particularly, the invention relates to a domain-wall-displacement-type magnetooptical recording medium having excellent recording/reproducing characteristics at a narrow track pitch and very small marks.
2. Description of the Related Art
Various magnetic recording media are used practically as rewritable recording media. Particularly, magnetooptical recording media in which information is recorded by writing magnetic domains in a magnetic thin film utilizing thermal energy of a semiconductor laser and the resulting recorded information is read using a magnetooptical effect are employed as large-capacity rewritable media allowing high-density recording. Recently, in accordance with the tendency toward digital recording of moving images, a need has arisen to provide large-capacity recording media by increasing the recording density of these magnetic recording media.
In general, the linear recording density of an optical recording medium greatly depends on the laser wavelength λ of a reproducing optical system and the numerical aperture (NA) of an objective. That is; since the diameter of a beam width is determined when the laser wavelength λ of the reproducing optical system and the numerical aperture (NA) of the objective are determined, the spatial frequency of a recording pit capable of reproducing a signal has a limit of about 2 NA/λ. Accordingly, in order to realize a high recording density in a conventional optical disk, it is necessary to shorten the laser wavelength of the reproducing optical system or increase the NA of the objective. However, it is difficult to shorten the laser wavelength because of problems in the efficiency and heat generation of a laser device, and the like. An increase in the NA of the objective requires high mechanical accuracy, for example, as a result of a decrease in the depth of focus.
Accordingly, various techniques for realizing high-density recording are being developed in which the recording density is improved by optimizing the configuration of a recording medium, or a reproducing method without changing the laser wavelength and the NA of the objective.
For example, in Japanese Patent Application Laid-Open (Kokai) No. 6-290496 (1994), a magnetooptical recording medium and a method for reproducing the same have been proposed in which, by displacing a domain wall present at a boundary portion of recording marks toward a higher temperature side by a temperature gradient and detecting the displacement of the domain wall, a signal having a recording density exceeding the resolution of an optical system can be reproduced without decreasing the amplitude of the reproduced signal. Such a reproducing method is called a DWDD (domain wall displacement detection) reproducing method. The configuration of the medium and the method for reproducing the medium are shown in FIGS. 21A–21C. In FIG. 21A, the medium includes a first magnetic layer 12001 having a small domain-wall coercive force, a second magnetic layer 12002 having a low Curie temperature, and a third magnetic layer 12003 having a large domain-wall coercive force. As described in “J. Magn. Soc. Jpn., 22, Suppl. No. S2, pp 47–50 (1998)”, the first magnetic layer 12001 operates as a displacement layer in which displacement of a domain wall occurs during a reproducing operation, the second magnetic layer 12002 operates as a switching layer for controlling the position of the start of displacement of the domain wall, and the third magnetic layer 12003 operates as a memory layer for holding information.
When a temperature distribution as shown in FIG. 21B is formed on these magnetic films, a domain-wall-energy-density distribution as shown in FIG. 21C is formed, so that a domain-wall driving force to displace domain walls toward a high temperature side having a low energy is generated. However, since these magnetic layers are subjected to exchange coupling at a region of temperatures lower than the Curie temperature of the switching layer, displacement of domain walls does not occur because of the large domain-wall coercive force of the memory layer, even when the domain-wall driving force is applied. On the other hand, since the exchange coupling force is weakened at a position of a temperature Ts near the Curie temperature of the switching layer, only domain walls in the displacement layer having a small domain-wall coercive force are displaced toward a high temperature side.
This displacement of domain walls is generated with a time interval corresponding to the spatial interval of the domain walls, when the medium is moved relative to the temperature distribution. Accordingly, by detecting the generation of displacement of domain walls, it is possible to generate a signal irrespective of the resolution of the optical system.
In conventional DWDD media, in order to allow smooth domain wall displacement during a reproducing operation, i.e., in order to mitigate the influence of a floating magnetic field at a temperature near a reproducing temperature where domain walls are displaced, the composition is designed so that the total magnetization of the displacement layer is substantially zero at a temperature (hereinafter represented by “Ts”) near the Curie temperature of the switching layer, i.e., a reproducing temperature. In such conventional media, there is no consideration given to total magnetization at a low temperature region from room temperature to reproducing temperature, or the influence of a random stray magnetic field from random magnetization present at an adjacent track in a low temperature region equal to or lower than Ts, or in front of and behind the track. It has not been previously recognized that these factors cause problems. Particularly, when intending to narrow the track pitch or provide a very small mark, the influence of a random stray magnetic field cannot be neglected. To do so results in degradation of recording/reproducing characteristics.
When intending to provide a very small recording mark length, a magnetic film having large magnetic anisotropy is used as the memory layer for holding information, which permits stable recording/holding of a further smaller recording mark. As a result, the energy required for magnetization reversal during a recording operation is greater. The intensity of a magnetic field that can be generated by a magnetic head in magnetic-field modulation recording suitable for high density recording is, at most, about 200–300 Oe (oersted in the CGS system of units), and is preferably equal to or less than 200 Oe during high-speed recording at low power consumption. Accordingly, for example, it is necessary to set the magnitude of magnetization of the memory layer to a more or less large value in the recording temperature range by adjusting the composition ratio or adopting a multilayer structure.
However, when adjusting magnetization at a region near the recording temperature range in the above-described manner, magnetization inevitably has a large value at a low-temperature region equal to or lower than Ts, which results in a greater influence thereon by the stray magnetic field during a reproducing operation.
In a land/groove substrate, as shown in FIG. 22, the height of a memory layer of a groove portion and the height of a displacement layer of a land portion of each of adjacent tracks have substantially the same value. This results in degradation of the recording-magnetic-field sensitivity by the influence of a stray magnetic field from the displacement layers of the adjacent tracks during a recording operation in a groove portion.
As described above, in a medium used in a DWDD reproducing method, balancing desired improvement in surface recording density, for example, by using very small marks and a narrow track pitch, while simultaneously improving recording/reproducing characteristics is very difficult.